This invention relates to air distribution systems, in particular apparatus for extracting air from a main supply duct to a branch duct.
It is well known to distribute air in a building from a main air supply duct to various branch ducts through openings in the wall of the main duct which enter into the branch ducts. The volume flow rate of air through the branch is determined to some extent by the static pressure in the main duct and the flow resistance of the branch. Because the branch opening is flush with the wall of the main duct in the commonly used distribution systems, the dynamic pressure of the air flow in the main duct does not assist the flow rate in the branch duct.
Attempts have been made to control or reduce the level of noise created by such air distribution systems. With the aforementioned configuration, the noise level at the start of the branch duct is generally the same as the noise level in the main duct, the noise being caused primarily by the air supply fan used in such systems. It is known to use a silencer at the exit of the fan in the main duct to reduce the noise level. Silencers have also been employed at the inlet to the main supply fan. In order that the silencer will not unduly affect the operation of the system, its use must result in a low pressure drop and its total open area must be sizable. Thus, the silencer must be relatively large. Because of this, known silencers can be costly and can require a large amount of space in the building.
U.S. Pat. No. 4,418,788, issued Dec. 6, 1983 to Mitco Corporation, describes a branch take-off and silencer for an air distribution system. The apparatus includes a static pressure regain section and a channel section adapted for coupling the input duct to an output duct and branch ducts. The inner surface of the wall of the regain section and that of the outer wall of the channel section form a continuous curve which results in smooth changes in air flow velocity in order to provide efficient conversion of velocity pressure to static pressure. A major difficulty with such an apparatus is that, due to the round cross-section of the take-off passageway, such an apparatus is difficult to manufacture, particularly if maximum efficiency is to be obtained. Also, aspects of this known design are not particularly helpful in reducing the noise level in the system or in the branch ducts.
U.S. Pat. No. 4,319,521, issued Mar. 16, 1982 to Mitco Corporation, describes an air distribution system that includes a mixing plenum for receiving and mixing outside and return air. There is an input flow concentrator, an integral silencer disposed within and coupled to the mixing plenum and this device is adapted to establish a substantially axially symmetrical flow path for air from the plenum to an output port. A fan is coupled to the output port to drive the air through the main duct for distribution. The path defining walls of the concentrator are lined with acoustically absorbent material.
It is an object of the present invention to provide an improved branch take-off air flow device for use in an air distribution system, which device is not unduly difficult to manufacture and which has improved sound attenuating capabilities.
It is a further object of the invention to provide a branch take-off air flow device particularly suited for use downstream of an axial fan having a central hub and a round fan outlet. This take-off device includes an elongate air flow defining member located centrally in the main air passageway and extending in the axial direction. The provision of this generally round member provides improved air flow characteristics to the branch take-off device and greater sound attenuation.